Will an 05-07 YZ125 engine fit in older YZ125's? If so, what years?

I was wondering if you could retrofit a newer 05-07 yz125 engine into the older steel framed models. They did a major rework of the engine(4 pounds lighter) as well as going with the aluminum frame in 05.

I am not sure how to find this information out. If someone knows or can point me in the right direction, I would appreciate it.

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On the 250's, the engines will interchange from 01-present. Not sure why you would want to retrofit. 4 lbs lighter engine going back into a steel frame is a bit odd. On the other hand, if you need a motor and there is one available, go for it.
I just saw a complete 250 motor on the auction site for $1500.

Somehow this thread got hijacked to a YZ250. It's cool cuz I love the 250. I ride YZ250's. I have owned every year from 2001-2005. I can tell you that the 05 YZ250 definitely feels lighter and more flickable than an 04. The 04 was/is my favorite bike but the 05 handles differently. It feels like the weight of the bike has a lower center of gravity. The first time I rode the 05 I noticed this immediately.

I really think the 05 YZ250 weighs less than an 02-04 YZ250. I think the published weight is 214 for a 2004 and 211 for a 2005. I also seriously think the center of gravity is lower on the 05-07.

Back to the 125's. The engine itself is over 4 pounds lighter for 05-07. The weight has definitely decreased on these bikes.

For the 250's, the engine stayed the same through the frame change from steel to aluminum. Not so with the YZ125. The engine went through a MAJOR change along with the frame change. Does this mean the newer engine will not retrofit to the steel frames? I do not know. I would like to know what years would work, if any.

I bought an used 01. PR what I thought was. A 01. But. It turned out to be a 01frame with a 05 engine. So after buying a 04 frame. And installing the engine I have found out. Yes it does. Fit. But. I had to. Dent. The back of my pipe to fit it around the engine basically have. 01 parts on the bike with a 04 frame and a 05 engine I did run into. A few minor. Issues. But. They are all a simple fix. Like the 01 plastics don't fit. As well as the seat. And. The 04 carb. Hole to engine is. Too big to. Connect. I do plan on buying a 05 frame tho. On the near future. But other than that. It all went together. Easily if anyone has any questions. Feel free to ask I know this is a older post